1. Field of the Invention
The invention relates to an FM-CW radar, which is used for preventing collision between moving bodies such as vehicles or adaptive cruise control (following at a fixed distance) of the moving body and which detects a relative velocity for a target existing outside of the moving body and a distance thereto, by means of radar transmitting/receiving.
2. Description of the Related Art
A radar for a vehicle has been desired as a device, which enables collision protection, automatic driving, and the like, from a viewpoint of driving safety and comfort. The FM-CW (frequency-modulated continuous-wave) radar transmits a radio wave with frequency-modulated (FM) at a predetermined cyclic period toward a target, and extracts a beat frequency component between a reflected wave having been reflected by the target and the transmission signal, to thereby obtain a distance to the target and a relative velocity for the target. Since such an FM-CW radar is simple in configuration of a transceiver, and is capable of obtaining a distance to the target and a relative velocity for an object by means of a simple signal processing device, the FM-CW radars are likely to come into extensive use as a radar for vehicles that are required to be compact and inexpensive.
The FM-CW radar is configured to calculate a distance to an object and a relative velocity for the object as follows. The FM-CW radar transmits as a radar wave a transmission signal frequency-modulated by a modulation signal of triangular waveform. The frequency of the transmission signal increases and decreases progressively. The FM-CW radar receives the radar wave having been reflected by a target. The FM-CW radar mixes the received signal with the transmission signal to generate a beat signal, A/D converts the beat signal and stores it in a memory. The FM-CW radar applies fast Fourier transform processing to the thus-stored data in the memory, and calculates the distance to the target and/or the relative velocity for the target from a result of the processing.
However, the calculation of the fast Fourier transform processing requires processing enormous quantity of data while heavily using product-sum operations. Therefore, it is impossible to write an output of the A/D conversion into a memory in process of reading/writing the enormous quantity of processed data. As a result, when a single memory is used for reading/writing the data stemming from the fast Fourier transform by means of a signal processing circuit and for writing the A/D-converted data, it is inevitable to conduct time division for the reading/writing of the data stemming from the fast Fourier transform and for the writing of the data output from the A/D converter. During the course of writing of the A/D converted data, the fast Fourier transform processing is suspended. During the course of the fast Fourier transform processing, the A/D conversion is suspended. As a result, full use of data processing capacity has been inhibited.
In order to solve this problem, JP-A-Hei.9-43343 has proposed providing a pair of memories for storing A/D conversion data alternately; and switching and controlling reading from/writing into the pair of memories in synchronization with calculation period of the fast Fourier transform by the signal processing circuit so that while A/D-converted output is being written into one of the memories, processing data of the signal processing circuit are read from/written into the other memory, thereby enhancing processing efficiency.